I have seen this problem on this board before. My 2 week old IS 350 Ultra white/Red F sport is in the shop for the rattle that feels to me to be under the car under the driver's seat. The dealer cannot figure out the problem after a day, so is keeping my new car. At leaset they were able to hear the rattle. Hope they can figure it out. Any advice would be appreciated.

 

Sounds to me like the common issue with the ebrake assembly. Have them grease it up and it should go away. If it rattles on things like lane dividers, little bumps, etc, then it's most likely that!